1. Field of the Invention
This invention relates to a high speed sampling circuit for analysis of fast signal waveforms, and more particularly to a circuit using two Josephson junction devices, one as a sampling pulse generator and one as an amplitude discriminator to obtain a very short sample of a repetitive waveform.
2. Background of the Invention
U.S. Pat. No. 3,983,419 issued to Fang entitled "Analog Waveform Transducing Circuit" provides an excellent discussion of the background for use of Josephson tunneling devices in sampling circuits.
One earlier approach mentioned is that of Zappe, U.S. Pat. No. 3,764,905 issued on Oct. 9, 1973, entitled "Apparatus for Measuring Pulsed Signals Using Josephson Tunneling Devices". In Zappe, two Josephson junctions were utilized. Each Josephson device was interconnected in a superconductive loop wherein the first loop and device captured a given quantity of magnetic flux quanta from a signal during a predetermined amount of time (termed "acquisition time"). The first loop and device could be switched between two states--one state termed "resistive" enabled the magnetic flux from the input signal to intercept the first loop and the second state is termed "non-resistive" which enabled the first loop to trap the intercepted flux thus providing a permanent measure of the signal level at the time of transition to the non-resistive state. The second loop and device was then utilized to measure the persistent current (which is proportional to the trapped flux) in the first loop.
In the Fang approach, only one Josephson junction device was utilized and it was connected in series with a signal source. The junction was further connected in a superconductive loop which contained distributed inductance. A control loop was utilized to switch the Josephson device between the "resistive" and the "non-resistive" states. In the "resistive" state, substantially all of the current from the signal source passed through the superconductive loop. When the current in the control loop was reduced to zero, the device switched to its superconductive state (i.e. the "non-resistive" state) and since the inductance in the device was smaller than the inductance in the circuit, the current flowing in the loop was maintained at its value. In Fang, the acquisition time (the time required for the superconducting circuit to be updated to the current value of the sampling current) was determined by the time constant L/R of the circuit which was estimated by Fang to be on the order of 10 picoseconds. The holding time (i.e. time during which the sample can be held at a constant value) in principle was indefinite.
The two above prior art approaches both rely upon sampling a signal by trapping magnetic flux in a superconducting loop containing a Josephson junction device. As pointed out in Fang and graphically shown in Fang's FIG. 20, it is highly desirable to have an acquisition time such that the measured signal can substantially track the applied analog signal. As previously mentioned, the acquisition time is limited by the L/R time constant of the superconducting loop. The critical shortcoming of these two approaches, therefore, is the time required for acquisition.
My approach overcomes this critical shortcoming in that a superconducting loop is not utilized and the acquisition time is determined solely by the switching time of the Josephson devices without any additional time required for flux to penetrate a loop. However, my approach is limited by the factor that many samples at a given point in the repetitive waveform must be made in order to resolve the analog signal level at that point. In comparison, the Zappe and Fang methods provide many bits of resolution from a single sample. However, in sampling oscilloscope applications, having a short acquisition time is generally more important than minimizing the number of required samples.
My approach is based upon the effect that a Josephson junction is capable of producing extremely short voltage pulses. The following references discuss this phenomena: Pierre Gueret, IEEE Trans. Magnetics, p. 751 (1975), Pierre Geuret, Applied Physics Letters, Vol. 25, p. 426, 1974 and C. S. Owen et al Physical Review, Vol. 164, p. 538-544, 1967. In fact, the higher the impedence connected to the Josephson device, the shorter in time the pulse will be. By utilizing one Josephson device to produce short pulses together with a second Josephson device acting as a latching type amplitude discriminator, a short sample of an analog signal can be obtained. The result is an apparatus that can be used in a sampling oscilloscope to achieve a resolution time of a few picoseconds.